The simplest way of stating Plato’s formation of his own doctrine is this: he accepted the Protagorean doctrine of a perceptual world of relative knowledge; he placed it beside the Socratic theory of conceptual reality; and as a result he conceived the world to be twofold. Both Being and Becoming share in reality. There are, on the one side, the immutable concepts that compose true reality; there are, on the other side, the changing perceptions that come and go. The world of true reality _is_, but never _becomes_; the world of relative reality _becomes_, but never _is_. These two worlds are by nature separate; one is the object of the reason, the other is the object of the senses; one is incorporeal, the other is corporeal. The first world is the immutable One of the Eleatics presented by Plato as a plural number of Socratic concepts; the other world is the Heracleitan flux presented as perceivable things. There is true knowledge, but Protagoras is right in saying that it cannot be found in the perception of the material world. It is knowledge of an incorporeal world, and that is precisely the world of Socratic concepts which now in Plato’s hands become Ideas. It would, however, be a mistake to suppose that Plato’s conception of the world was an artificial eclecticism, obtained by putting two worlds side by side. To be sure, he never was able to bring them into an organic unity, and the dualism between them is often very marked. But they do not lie like two drawers in a desk, each having no vital influence on the character of the other. In the juxtaposition of the two worlds each gets a new meaning, and the value of each becomes greater. In the first place, _perception[30] gets a new value_. The logic of the Sophistic doctrine of perception was that perceptions are the only form of knowledge, and even perceptions have no share of truthfulness. Protagoras himself did not go so far as this absolute skepticism, but this is the logic of his position. Perceptions can have no value, because each is a standard to itself. Plato incorporates the perception theory into his own, and immediately gives it a new value. Perceptions do not, to be sure, yield true knowledge, but they have a relative value. They have a value for the practical world, although the highest they can give is Right Opinion. When we remember that the world of that day was weary of its own speculations leading to nihilism, it is remarkable that Plato did not turn away entirely from the doctrine of the Sophists. On the contrary, he took up the Sophistic doctrine into his own and gave to it a value which it had not possessed by itself.